“It is presented as a magic solution but handing over whole sectors of our economy to be run by politicians would be hugely expensive to the public finances and wouldn’t necessarily improve the experience of all of us who use and rely on these services, day in and day out.

“Moreover, it could be particularly damaging for millions of pensioners who have saved for their retirement for decades.”

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The Nera report is a fresh blow for Mr Corbyn’s plans and comes after a recent ComRes poll showed public support for nationalisation had halved over the past nine months from 83 per cent to just 42 per cent.

Labour described the figures as an “absurd” attempt by water companies to protect their monopolies.

A spokesman said: “Instead of wasting their money on dodgy dossiers to instigate a project fear campaign to protect their rip off rates they should be using that money to keep customers’ bills down.

“Labour will fix this broken system by bringing the water companies back into public ownership, saving households £100 per year on their bills.”

He added: “The firm they have paid to create these fantasy figures to frighten pensioners with is the same one that’s received heavy critism for their work that was cited by Donald Trump to pull out of the Paris Agreement undermining global efforts to tackle Climate Change.”